Reversible window unit for alternately reflecting and absorbing solar energy

ABSTRACT

A multiple layer coating is disclosed which enables a window unit to function efficiently as a solar energy collector in winter and as a heat shield in summer. The coating comprises a layer of metal which provides high reflectance of incident solar energy from one side of a coated transparent substrate and a layer which provides high absorption of incident solar energy by the other side of the coated substrate. The absorbing layer may be a single film of a highly absorptive material, but is preferably a dual film of an absorptive material covered by an antireflective material. The absorptive layer may also comprise multiple alternating films of metal and dielectric materials which effectively absorb incident solar energy. The reflective and absorptive layers are selected to provide the coated article with a low U-value to prevent heat radiation from the warm building to the cold environment in winter and from the hot environment to the cooled building in summer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to coated glass articles andmore particularly to a multiple layer coating on a transparentsubstrate, especially in a multiple glazed window unit.

2. The Prior Art

More efficient use of energy in the heating and cooling of residentialand commercial buildings can be achieved by employing insulating windowunits, especially those with coated surfaces which selectively reflect,transmit or absorb incident solar energy.

In U.S. Pat. No. 3,925,945, White discloses a transparent insulatingbuilding panel having a pivotally mounted frame enclosing a heatabsorbing glass pane spaced from an insulating panel comprising twopanes of clear glass separated by a clear air space and includingapertures in the frame providing free air paths through the spacebetween the heat absorbing glass and the insulating panel.

In U.S. application Ser. No. 791,350, filed on Apr. 27, 1977, Franzdiscloses a seasonally adjustable window including four spaced glasssheets, the outer sheets being selectively coated to provide a lowshading coefficient in the summer position, a higher shading coefficientand a 50 percent reflectance of low temperature radiation in the winterposition, and a low U-value.

In U.S. Pat. No. 3,990,784, Gelber discloses an architectural glasssystem having a multilayer coating comprising first and second metallayers and a dielectric material such as silicon dioxide, magnesiumfluoride or aluminum oxide disposed between the metal films. Gelberteaches that the transmission of the coating can be changedindependently of its reflection properties by varying the individualthicknesses of the metal layers while maintaining the ratio of the layerthicknesses constant.

In "Solar Absorptance and Thermal Emittance of Evaporated Coatings",Physics of Thin Film, Vol. 2, pp. 305-361, Drummeter and Haas discusssurface properties and temperature control in relation to objects inspace and solar energy conversion. Silicon oxide and aluminum oxidefilms on aluminum substrates as front surface mirrors for satellites aredisclosed.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view illustrating a reversible window unit 10pivotally mounted 20 in a wall opening 30.

FIG. 2 is a partial cross-sectional view of a seasonally adjustabledouble glazed window according to the present invention in the summerposition. Incident solar energy transmitted through a transparentsubstrate 11 is largely reflected by a layer of metal 12.

FIG. 3 is similar to FIG. 2 but shows the unit in the winter position.Incident solar energy transmitted through transparent substrate 15 andspace 14 is largely absorbed by coating layer 13. A significant portionof the energy is conducted through the metal coating layer 12 andsubstrate 11 to the interior space of the building.

SUMMARY OF THE INVENTION

The present invention involves a novel coated article comprising atransparent substrate, a coating layer which reflects solar energy, anda coating layer which absorbs solar energy. The reflective coating layeris a metal film with high reflectance such as aluminum, chromium,platinum or molybdenum. The absorptive layer may be a single film of amaterial with high absorptance such as germanium or silicon. However,the absorption of such materials is preferably enhanced by an additionalfilm of an antireflective material such as silicon oxide or magnesiumfluoride. The absorptive layer may also comprise alternating films ofmetal and dielectric materials which effectively absorb incident solarenergy. The coated article is useful as a window and can be employed asa solar heat shield by facing the metal film toward the sun or as asolar energy collector by facing the absorbing layer toward the sun. Thecoated article is most useful as a pane in a double glazed window unitwhich is reversible so that the functions may be alternatinglyperformed. The metal film provides maximum reflection of solar energywhen the unit is in the summer position and the absorbing layer permitsmaximum absorption of solar energy when the unit is in the winterposition. The multiple layer coating of the present invention has a lowemissivity in the room temperature range of the radiation spectrum andprovides the unit with a low U-value. This prevents heat radiation fromthe warm building to the cold environment in winter and from the hotenvironment to the cooled building in the summer.

Coatings useful according to the present invention, that is, coatingswhich result in high absorption by the coated side, high reflection fromthe glass side, and a low U-value, include aluminum/silicon/magnesiumfluoride, aluminum/germanium/silicon monoxide, aluminum/silicondioxide/chromium/silicon dioxide and others.

Optimum performance is obtained with a multiple layer coating ofaluminum/germanium/silicon monoxide on glass. The aluminum provides highreflection from the glass side of the coated article. The germaniumprovides high absorption by the coated side. The silicon monoxideprovides antireflective properties which enhance the absorptioncapability of the germanium layer. An additional feature of thiscombination of coating layers is that the color of a double glazed unitas observed through the uncoated pane can be changed from yellow togreen to red to blue simply by varying the thickness of the siliconmonoxide layer without significantly affecting the solar energyperformance of the unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is a multiple layer coating preferably on theinside surface of one pane of a reversible double glazed window unitwhich enables the unit to perform with maximum efficiency as a heatshield in summer and as a heat collector in winter. The multiple layercoating comprises a layer of metal preferably deposited on a glasssurface which faces the enclosed space in the unit. This layereffectively reflects a significant portion of incident solar energy whenthe coated glass is the outside pane in the double glazed unit. Thepreferred metals include aluminum, chromium, platinum, or molybdenum,but more commonly deposited metals such as silver, gold or copper mayalso be used. The coating further comprises a layer which effectivelyabsorbs a significant portion of incident solar energy when the coatedglass is the inside pane in the double glazed unit. The absorbing layermay be a single film of a material with high absorptance such assilicon, germanium and other semiconducting materials having anabsorption edge between 1 and 4 microns. However, it is preferred toemploy an antireflective film over the silicon or germanium in order toenhance the absorption capability. Such materials as silicon oxides,aluminum oxides, magnesium fluoride and other dielectric materialsprovide such antireflective properties while transmitting the infraredportion of the incident solar radiation for absorption by the silicon orgermanium. The absorbing layer may also comprise multiple alternatingfilms of metal and dielectric materials which effectively absorbincident solar energy. Solar radiation absorbed by the absorbing layerof the coating is transferred primarily by conduction through the metallayer and the glass substrate to the interior of the building.

In a most preferred embodiment of the present invention, a sheet oftypical soda-lime-silica glass is coated by vacuum deposition. Thesubstrate is placed in an air-tight coating chamber and the chamber isevacuated to below 5×10⁻⁴ Torr, preferably below 1×10⁻⁴ Torr. Thesubstrate temperature is typically ambient but may be elevated for thedeposition of some materials, such as magnesium fluoride, to about 300°C. The maximum temperature for the substrate is limited by its softeningpoint. The first coating layer, preferably aluminum, platinum ormolybdenum, and most preferably aluminum, is deposited by evaporatingthe metal from a suitable source such as a heated tungsten strip or awater-cooled electron beam gun crucible and condensing the evaporatedmetal on the substrate. Deposition of the metal is preferably continueduntil a luminous reflectance from the coated surface of about 50 to 60percent is obtained. The second coating layer is preferably a dual filmof high absorptance germanium or silicon with antireflective siliconoxide or magnesium fluoride. The high absorptance material is evaporatedin the same manner as the metal and condensed on the metal film.Deposition is preferably continued until a reflectance from the coatedsurface of about 25 percent is obtained. Finally, the antireflectivematerial, preferably silicon oxide, is evaporated in like manner andcondensed on the absorptive material. Deposition is continued until thereflectance from the coated surface is very low, preferably about 1percent or less. The thicknesses of the coating layers are preferablysuch that the final luminous transmittance of the coated article is atleast about 5 percent and preferably in the range of 8 to 20 percent.

The multiple coated glass has high reflection from the glass surface dueto the reflective metal layer and high absorption by the coated surfacedue to the combination of the high absorptance germanium layer with theantireflective silicon monoxide layer. The reflectance from the glassside is preferably at least about 40 percent while the absorption by thecoated surface is at least about 40 percent and is preferably more than60 percent. The multiple coated glass surface preferably has a lowemissivity in the low temperature radiation portion of the spectrum, 2to 50 microns, to prevent the radiation of heat from the warm buildingto the cold environment in the winter and from the hot environment tothe cooled building in the summer. In other words, the coating shouldgive the unit a low shading coefficient, preferably less than about 0.6,and a low U-value, which is the overall coefficient of thermal transfer,preferably less than about 0.5 BTU/hour-square foot-°F. (about 2.8watts/square meter-°C.).

The present invention will be further understood from the description ofa specific example which follows.

EXAMPLE I

A sheet of 90 mil (about 2.25 millimeters) thick soad-lime-silica glassis vacuum coated with a first layer of aluminum, a second layer ofgermanium and a third layer of silicon monoxide to a final lunimoustransmittance of about 20 percent. The monolithic multiple coated glasshas the following properties. Total luminous reflectance from the glasssurface (summer position) is about 55 percent. Absorption by the filmsurface (winter position) is about 79 percent. The monolithic multiplecoated glass sheet has a summer shading coefficient, which is the ratioof its solar heat gain to the solar heat gain of monolithic clear sheetglass, of 0.28.

The above example is offered to illustrate the present invention.Variations and modifications of the above are included in the scope ofthe invention. For example, the first coating layer may be a metal otherthan aluminum, such as chromium, platinum, molybdenum, silver, gold orcopper, so long as it imparts the desired reflectance properties to theunit. Likewise, the exterior coating layer may be an antireflectivematerial other than silicon oxide, such as alumina or magnesium floride.The high absorptance layer may be a semiconducting material other thangermanium or silicon so long as the desired absorption is obtained. Theabsorbing layer may comprise multiple alternating reflective andantireflective films which effectively absorb incident solar energyrather than a film of high absorptance material. For example, multiplelayer coatings such as Al/SiO/Ge/SiO, Al/SiO/Cr/SiO, Pt/SiO₂ /Pt/SiO₂,and Al/SiO/Al/SiO are included. The scope of the present invention isdefined by the following claims.

I claim:
 1. A coated article for alternately reflecting and absorbingincident solar energy comprising:a. a transparent substrate; b. areflective coating layer of metal, deposited on a surface of saidtransparent substrate; c. an absorptive coating layer, deposited on saidmetal; and d. an antireflective material selected from the groupconsisting of silicon oxides, aluminum oxides and magnesium fluoridedeposited on said absorptive coating layer.
 2. A coated articleaccording to claim 1, wherein the substrate is glass.
 3. A coatedarticle according to claim 1, wherein the metal is selected from thegroup consisting of aluminum, chromium, platinum, molybdenum, silver,gold and copper.
 4. A coated article according to claim 1, wherein saidabsorptive coating layer comprises a semiconducting material having anabsorption edge between about 1 and 4 microns.
 5. A coated glass articleaccording to claim 1, wherein the article has a luminous transmittanceof at least about 0.5 percent, a reflectance from the uncoated surfaceof the substrate of at least about 40 percent and an absorption by thecoated surface of at least about 40 percent.
 6. A coated article foralternately reflecting and absorbing incident solar energy comprising:a.a transparent substrate; b. a reflective coating layer of metaldeposited on a surface of said substrate; and c. an absorptive coatinglayer which comprises a material selected from the group consisting ofgermanium and silicon deposited on said metal.
 7. A coated article foralternately reflecting and absorbing incident solar energy comprising:a.a transparent substrate; b. a reflective coating layer of metaldeposited on a surface of said substrate; and c. an absorptive coatinglayer which comprises alternating layers of metal and dielectricmaterials which effectively absorb incident solar energy deposited onsaid metal.
 8. A multiple glazed window unit for alternately reflectingand absorbing a significant portion of incident solar energycomprising:a. at least two transparent substrates in facingrelationship; b. a reflective metal coating deposited on one of thesubstrates on a surface facing another substrate; and c. an absorptivecoating, deposited on said metal coating, which absorbs more andreflects less incident solar energy than said metal coating, and whichcomprises, as its exposed surface, an antireflective material selectedfrom the group consisting of silicon oxide, aluminum oxide and magnesiumfluoride.
 9. A window unit according to claim 8, wherein the transparentsubstrates are glass.
 10. A window unit according to claim 9, whereinthe unit has a luminous transmittance of at least about 5 percent, areflectance from the glass surface of at least about 40 percent and aU-value less than about 0.5 BTU per hour-square foot-°F.
 11. A windowunit according to claim 8, wherein the metal is selected from the groupconsisting of aluminum, platinum, chromium, molybdenum, silver, gold andcopper.
 12. A window unit according to claim 8, wherein the absorptivecoating comprises a semiconducting material with an absorption edge inthe range of 1 to 4 microns.
 13. A window unit for alternatelyreflecting and absorbing incident solar energy comprising:a. at leasttwo transparent substrates in facing relationship; b. a reflective metalcoating deposited on one of the substrates on a surface facing anothersubstrate; and c. an absorptive coating which comprises alternatinglayers of metal and dielectric materials which cooperate to effectivelyabsorb incident solar energy.
 14. A window unit for alternatelyreflecting and absorbing incident solar energy comprising:a. at leasttwo transparent substrates in facing relationship; b. a reflective metalcoating deposited on one of the substrates on a surface facing anothersubstrate; and c. an absorptive coating which comprises an absorptivematerial selected from the group consisting of germanium and silicondeposited on said metal coating.
 15. A reversible multiple glazed windowunit for alternately reflecting and absorbing incident solar energycomprising:a. at least two transparent substrates in facingrelationship; b. a reflective metal coating deposited on one of thesubstrates on a surface facing another substrate; c. an absorptivecoating, deposited on said metal, which reflects less and absorbs moreincident solar energy than said metal, and which comprises, as itsexterior surface, an antireflective material selected from the groupconsisting of silicon oxide, aluminum oxide and magnesium fluoride; d.frame means for enclosing the substrates in an integral unit; and e.means for pivotally mounting the unit in a wall opening so that the unitmay be rotated from one orientation to the opposite orientation.
 16. Areversible window unit according to claim 15, wherein:a. two transparentsubstrates are glass in spaced facing relationship; b. the metal coatingis selected from the group consisting of aluminum, platinum, chromium,germanium, molybdenum, silver, gold and copper; c. the absorptivecoating comprises an absorptive material selected from the groupconsisting of semiconducting materials having an absorption edge betweenabout 1 and 4 microns; and d. the unit has a luminous transmittance ofat least about 6 percent and a reflectance from the glass surface of thecoated substrate of at least about 40 percent.
 17. A reversible windowunit for alternately reflecting and absorbing incident solar energycomprising:a. at least two transparent substrates in facingrelationship; b. a reflective metal coating selected from the groupconsisting of aluminum, platinum, chromium and molybdenum deposited on asurface of a substrate facing another substrate; c. an absorptivecoating which comprises a material selected from the group consisting ofgermanium and silicon deposited on said metal; d. frame means forenclosing the substrate in an integral unit; e. means for pivotallymounting the unit in a wall opening so that the unit may be rotated fromone orientation to the opposite orientation;wherein the unit has aU-value less than about 0.5 BTU per hour-square foot-°F. and a shadingcoefficient less than about 0.6 and further comprises means for sealingthe edges of the substrate to provide a moisture-free air space betweenthe substrates.